The invention relates to a method of manufacturing a solid perform for drawing optical fibers. The method starts with a hollow preform having a central duct extending in the longitudinal direction of the hollow preform. At least the part of the hollow preform bounding the duct consists of quartz glass doped with a dopant which is more volatile than silica. In the method, the hollow preform is heated, while passing a fluorine-containing gaseous etchant through the duct, to a temperature at which the duct starts closing spontaneously so as to form a solid preform of the same length as the hollow preferom. In its specific embodiment, the invention is a method of manufacturing a solid optical preform, the method consisting essentially of the steps of providing a hollow optical fiber preform having a longitudinal direction, said hollow preform having an inside wall bounding a central duct, the central duct extending in the longitudinal direction, the inside wall portion of the hollow preform comprising quartz glass doped with a dopant, said dopant being more volatile than quartz glass; heating the hollow preform to a temperature at which the duct closes spontaneously to form a solid preform; and pasing a gaseous etchant through the central duct while the duct is closing and without the formation of a plasma in the duct, said gaseous etchant consisting of oxygen and C.sub.2 F.sub.6.
Hollow preforms which have a central duct extending in the longitudinal direction occur as intermediate products in various proceses for manufacturing optical fibers. For example, such hollow preforms may be produced in a process in which vitreous particles are deposited on a mandril, until the desired quantity of material has been deposited, after which the mandril is removed. In other processes (e.g. MCVD and PCVD) quartz glass tubes are coated internally with vitreous material to produce hollow preforms.
Nowadays it is common practice to close the longitudinal duct in a separate manufacturing step before starting to draw the fiber. This presents important advantages over processes in which fibers are drawn directly from hollow preforms. Solid preforms can be stored, for example, for longer periods of time without contaminating the inner layers which will be come the light-propagating cores of the fibers.
When closing the central duct, the procedure is, for example, as follows. The preform is heated by a burner which is reciprocated at a uniform speed along the length of the rotating preform. Under the influence of the surface tension of the softened preform material, the diameter of the duct gradually decreases until the duct closes entirely on passing the burner for the last time. This method is used in particular in closing hollow preforms obtained by means of an MCVD or PCVD process.
In this case of porous preforms which are constructed from vitreous particles, the duct closes during the consolidation or vitrification of the particles.
A generally used dopant in the manufacture of optical fibers from quartz glass is germanium dioxide (GeO.sub.2). When collapsing preforms containing quartz glass doped with a comparatively volatile dopant, for example germanium dioxide, a part of the dopant may evaporate from layers adjacent the central duct. This results in a disturbance in the refractive index profile. In this case, the diffusion of the dopant from parts of the preform situated further away from the central duct also plays a part. The profile disturbance has an adverse influence on the bandwidth of the optical fiber.
It has already been suggested to reduce the profile disturbance by flowing a fluorine-containing gaseous etchant through the central duct when collapsing the hollow preform. The gaseous etchant reacts with the layer which has been depleted of dopant by evaporation, while forming gaseous etching products. (See, for example, GB No. 2,084,988 A and FR No. 2,504,514). Gaseous etchants mentioned are mixtures of oxygen and dichlorodifluoromethane (CCl.sub.2 F.sub.2), sulphur hexafluoride (SF.sub.6), carbon tetrafluoride (CF.sub.4), trichloromonofluoromethane (CCL.sub.3 F) and monochlorotrifluoromethane (CCLF.sub.3).
It has been found in practice that the known etchant compounds do not give satisfactory results. It has been found tht the disturbance of the refractive index profile can be reduced, but it cannot be avoided. (See, French patent application No. 2,504,514).